According to the prior art, automatic transmissions, particularly for motor vehicles, comprise planetary gear sets that are shifted using friction elements or shift elements such as clutches and brakes, and typically are connected to a start-up element, such as a hydrodynamic torque converter or a fluid coupling, that is subject to a slip effect and is provided optionally with a lock-up clutch.
Such an automatic transmission is known, for example, from DE 199 12 480 B4 by the applicant. It comprises three single-carrier planetary gear sets, as well as three brakes and two clutches for shifting six forward gears and one reverse gear, a input shaft and an output shaft, wherein the carrier for the first planetary gear set is continuously connected to the ring gear of the second planetary gear set, and the carrier for the second planetary gear set is continuously connected to the ring gear of the third planetary gear set, and the input shaft is directly connected to the sun gear of the second planetary gear set.
Furthermore, in the known transmission, the input shaft is connectable, by means of the first clutch, to the sun gear of the first planetary gear set and, by means of the second clutch, to the carrier of the first planetary set, wherein the sun gear of the first planetary gear set is connectable, by means of the first brake, to a housing of the transmission, and the carrier of the first planetary gear set is connectable, by means of the second brake, to the housing of the transmission, wherein the sun gear of the third planetary gear set is connectable, by means of the third brake, to the housing of the transmission. The input shaft of the transmission is continuously connected to the carrier for the third planetary gear set and the ring gear of the first planetary gear set.
Furthermore, a nine-speed multi-stage transmission is known from DE 29 36 969 A1; it comprises eight shift elements and four planetary gear sets, wherein one planetary gear set serves as a front-mounted transmission and the main gearing includes a Simpson set and a further planetary gear set serving as a reverse gearing.
Further multi-stage transmissions are known, for example, from the applicant's DE 10 2005 010 210 A1 and DE 10 2006 006 637 A1.
Automatically shiftable vehicle transmissions of a planetary design are already generally described numerous times in the prior art and are continually undergoing further development and improvement. These transmissions should have a relatively simple design, in particular requiring a low number of shift elements, and minimize the need for double shifting, i.e., engaging and/or disengaging two shift elements, when sequential shifting is performed, thereby ensuring that only one shift element is ever switched when shifting is performed in defined groups of gears.
The applicant's document, DE 10 2008 000 428 A1, discloses a multi-stage transmission of a planetary design that includes an input drive and an output drive which are disposed in a housing. In the known transmissions, at least four planetary gear sets, referred to in the following as the first, second, third and fourth planetary gear sets, at least eight rotatable shafts, referred to in the following as the drive shaft, output shaft, third, fourth, fifth, sixth, seventh and eighth shafts, as well as at least six shift elements, comprising brakes and clutches, are provided, the selective engagement of which produces different transmission ratios between the input drive and the output drive, so that preferably nine forward gears and one reverse gear can be implemented.
The first and second planetary gear sets, which are preferably designed as minus planetary gear sets, thus having a negative stationary transmission ratio, form a shiftable front-mounted gear set, wherein the third and fourth planetary gear sets form a main gear set.
In the known multi-stage transmission, the carriers of the first and second planetary gear sets are coupled together via the fourth shaft which is connected to an element of the main gear set, the ring gear of the first planetary gear set is coupled to the sun gear of the second planetary gear set via the eighth shaft, which is detachably connectable to the drive shaft via the first clutch, and the sun gear of the first planetary gear set can be coupled to a housing of the transmission by means of the third shaft, via a first brake, and is detachably connectable to the drive shaft via a second clutch, wherein the ring gear of the second planetary gear set can be coupled to a housing of the transmission by means of the fifth shaft via a second brake. In addition, the seventh shaft is permanently connected to at least one element of the main gear set, and can be coupled to the housing of the transmission via a third brake, and the sixth shaft is permanently connected to at least one further element of the main gear set and is detachably connectable to the drive shaft, via a third clutch; the output shaft is permanently connected at least to one further element of the main gear set.
In the known transmission, the fourth shaft is preferably permanently connected to the ring gear of the third planetary gear set, the sixth shaft is permanently connected to the ring gear of the fourth planetary gear set and to the carrier of the third planetary gear set, and is detachably connectable to the drive shaft via the third clutch. Furthermore, the seventh shaft is permanently connected to the sun gears of the third and fourth planetary gear sets, and can be coupled to a housing of the transmission, via the third brake. In this case, the output drive is produced via the output shaft that is permanently connected to the carrier of the fourth planetary gear set. Furthermore, the third and fourth planetary gear sets can be combined or reduced to a Ravigneaux set having a common carrier and a common ring gear.
According to the prior art, the shift elements of multi-stage transmission thus designed, which are usually designed as lamellar clutches or brakes, are actuated hydraulically, which disadvantageously results in significant hydraulic losses. In order to avoid these actuation losses, the use of shift elements that can be actuated upon demand would be especially advantageous.
Shift elements that can be actuated upon demand may be understood, in particular, to be shift elements for maintaining the shift state, thus which use no energy or less energy when disengaged or engaged than when switching the shift state.
In order to make it possible to use shift elements that can be actuated upon demand, the shift elements, in particular the clutches, should be easily accessible from outside.